Remove the need for PPX_BUILD_XR in project level code

For example in graphics benchmark app: https://github.com/google/bigwheels/blob/main/benchmarks/graphics_pipeline/GraphicsBenchmarkApp.cpp having to add PPX_BUILD_XR complicates code unnecessarily.
We should define the XR functions with or without PPX_BUILD_XR and make sure they behave appropriately if PPX_BUILD_XR=0, we can still have PPX_BUILD_XR in application.cpp.